Conjugates were prepared by carbodiimide-mediated coupling of adipic acid dihydrazide derivatives of Haemophilus influenzae type b and pneumococcus type 6A with tetanus toxoid. These conjugates were characterized by chemical, chromatographic and immunological methods. Their immunogenicity was assessed in mice and adelescent and infant rhesus monkeys as single preparations and in combination. The effect of simultaneous immunization of these conjugates with DTP was also studied. The conjugates were found to induce both anticarrier protein and antipolysaccharide antibodies in mice and rhesus monkeys including infant rhesus. The antipolysaccharide response elicited by the conjugates could be primed with the carrier protein and boosted by subsequent immunizations. Hib and Pu6A were shown to be cross-immunogenic and to enhance each others immune response when injected simultaneously. Antibodies induced by the conjugates had bactericidal, protective and toxin neutralization activities.